A 1,500-year environmental archive
The Kalawewa reservoir is a massive irrigation work completed around 460 CE under King Dhatusena to supply water to the ancient capital of Anuradhapura. This artificial lake, which has a capacity of 123 million cubic meters, was created by damming the Kala Oya river and connecting it via the 87-kilometer-long Jaya Ganga canal to the city's reservoirs. For centuries, sediment has washed into the reservoir, settling on the bottom in fine layers. Each layer contains a snapshot of the surrounding environment from the year it was deposited.
By drilling down and extracting a long cylinder of this mud (a sediment core) scientists can read this history page by page. The core from Kalawewa provides a continuous environmental record stretching back to the reservoir's construction during the Anuradhapura Kingdom. This mud contains microscopic clues about the landscape: pollen grains, charcoal fragments, and mineral signatures that record the history of deforestation, agriculture, and societal stress.
Signals in the sediment
Analysis of the Kalawewa sediment core shows the landscape transformation. In the deepest, oldest layers from the 5th century, pollen from native dry-zone forest species is abundant. As time progresses up through the core, these tree pollens decrease while pollens from grasses and agricultural crops increase. This change signals the clearing of forests to expand rice cultivation, the economic basis of the Anuradhapura Kingdom.
The core also contains microscopic fragments of charcoal, evidence of fire. The concentration of this charcoal rises in layers corresponding to periods of increased agricultural activity, indicating that fire was a primary tool for clearing land. Alongside these changes, geochemical analysis of the sediment shows a rise in elements associated with soil erosion. As forests on the surrounding hills were cut down, more topsoil washed into the reservoir during monsoon rains. This sediment infill threatened the long-term viability of the very irrigation system it was a part of.
These environmental pressures—deforestation and soil erosion—intensified in the centuries leading up to the 10th and 11th CE. The sediment record shows this ecological stress peaked just before the historical record notes the Anuradhapura Kingdom's decline and eventual collapse, which was marked by invasions and the capital's relocation. The mud of Kalawewa suggests the kingdom's foundations were eroding long before its final fall.